The present invention relates to mixing of buoyant particle slurries and more particularly to baffle designs for incorporation into mixing tanks which accomplishes the same.
Generally the effective mixing of buoyant particle slurries is very important to many process operations, for example, in the manufacture of elastomers. In the later process, synthesized polymer particles are fed to finishing sections as water slurry and holding tanks are then used to receive the slurry and supply it as uniform feed to product extruders. The mixing of these buoyant particles also is an important operation during other aspects of chemical manufacture, such as during the dissolving of polymer particles in processing lubricating oil additives. Heretofore, good mixing has not always been achieved and often times stagnant particles can agglomerate on the liquid surface which can cause system shutdowns. Another problem is that where the feed concentration is related to the tank level, then the receiver of the feed can be severely affected if the tank level significantly changes. Thus, it is clearly desirable and essential to have an effective buoyant particle mixing system which will prevent accumulation of stagnant particles at liquid surfaces and produce continuous flow outlet from the holding tanks at a concentration which does not vary significantly with changes in the tank liquid level.
Previous attempts have included employing holding tanks which are unbaffled and having a central vortex to provide the desired mixing. However, it has been found that various deficiencies exist in unbaffled tanks including the need for higher mixing speeds than normally anticipated; concentration of particles between the bottom of the vortex and top of the vortex-creating turbine, and undesirable surface waves at certain mixer speeds, especially in larger holding tanks.
Present mixing designs which include baffling typically have employed full baffles, i.e. baffles which extend for the full length of the tank walls at spaced locations. A typical fully-baffled mixing tank normally comprises four full length wall baffles each of which has a radial width of 1/10 to 1/12 of the tank diameter. This type of full baffling eliminates tank swirl and vortex formation, except when the liquid surface is very close to the top of an agitator.
Other prior art illustrative of employing full length baffles including U.S. Pat. Nos. 706,473 and 2,628,801. Still further mixing arrangements wherein the baffles are situated other than at or just below the liquid surface are disclosed in U.S. Pat. Nos. 1,353,166; 2,031,590; and 3,660,244 all of which suggest or disclose locating the baffles at or near the bottom of the tank. U.S. Pat. No. 2,143,652 discloses the location of the top baffles at the center of the tank and no vortex is formed. The latter expedient also is taught by U.S. Pat. No. 3,414,240 which discloses a vertically arranged finger baffle structure for suppressing vortices but which still results in a modified central vortex. U.S. Pat. No. 3,473,790 discloses centrally located annular baffles about the agitator at the liquid surface for intercepting flow from the agitator above the liquid surface directing it down without forming a vortex. Those patents which are typical of unbaffled arrangements are U.S. Pat. Nos. 905,025; 1,771,321; 2,042,818; 2,831,418; and 2,875,897, all of which result in undesirable regions of relatively high particle concentration within the mixing vessel. Those baffle arrangements which suppress or eliminate vortex formation can be found in U.S. Pat. Nos. 706,473; 1,353,166; 1,354,489; 1,756,236; 2,031,590; 2,628,081; 2,928,665; and 3,473,790; however, this type of arrangement makes particle removal from the liquid surface difficult.